1. Field of the Invention
The present invention relates to a hydraulic driving apparatus for a wheeled vehicle, which comprises a hydrostatic transmission (hereinafter referred to as an “HST”) for steering, i.e., for applying oppositely directive rotational forces to respective right and left drive wheels so as to turn the vehicle, wherein the HST is so constructed that a steering operational direction constantly corresponds to right-and-left turning direction of the vehicle whether the vehicle travels forward or rearward.
2. Background Art
Conventionally, as disclosed in Japanese Laid Open Gazette No. 2000-25,637, there is a well-known driving apparatus of a wheeled vehicle such as a tractor, which comprises a pair of HSTs, i.e., one for traveling (a traveling HST) and the other for steering (a steering HST), each of which is a fluid connection of hydraulic pump and motor through a pair of oil passages, wherein output rotational direction and speed of the hydraulic motor in each of the HSTs is decided by operating a correspondent one of traveling operation means and steering operation means (usually, the capacity and discharging direction of each of the hydraulic pumps is set in correspondence to the location of a movable swash plate of the hydraulic pump by the operation so as to decide the output rotational speed and direction of the hydraulic motor). For turning the vehicle, left and right axles are driven at a common speed and in a common direction by the hydraulic motor in the traveling HST (the traveling motor) and are driven at different speeds or in opposite directions by output of the hydraulic motor in the steering HST (the steering motor). Further, the hydraulic driving apparatus includes means which changes the output rotational direction of the steering motor according to switching the setting of the traveling operation means between forward traveling mode and rearward traveling mode so that the vehicle turns to left or right side coinciding with the setting side of the steering operation means whether the vehicle travels forward or rearward.
To serve as the means for changing the output rotational direction of the steering motor, Japanese Laid Open Gazette No. 2000-25,637 discloses a mechanical switching device interposed on a link mechanism among the traveling operation means, the steering operation means and a movable swash plate of the hydraulic pump in the steering HST (the steering pump). However, such a mechanical switching device is complicated and expanded.
Japanese Patent No. 3,176,237 discloses a hydraulic switching valve, which is electrically controlled based on operation of the traveling operation means and the steering operation means. The switching valve is interposed across the pair of oil passages between the hydraulic pump and the hydraulic motor in the steering HST.
The switching valve disclosed in Japanese Patent No. 3,176,237 has four ports and is switched among a neutral position, a forward traveling position, and a rearward traveling position. The switching valve is located at its neutral position by setting the traveling operation means into neutral as well as setting the steering operation means into neutral, where the switching valve separately divides the pair of oil passages between the steering pump and the steering motor into a pair of oil passages on the steering pump side and a pair of oil passages of the steering motor side so as to stop the steering motor, thereby surely keeping the vehicle stationary. When the traveling operation means is set for forward traveling of the vehicle, the switching valve is located in its forward traveling position where the pair of oil passages on the steering pump side are connected to the respective oil passages on the steering motor side. When the traveling operation means is set for rearward traveling of the vehicle, the switching valve is located in its rearward traveling position where the oil passages on the steering motor side to be connected to the respective oil passages on the steering pump side are exchanged.
However, Japanese Patent No. 3,176,237, disclosing an agricultural combine having a pair of right and left sprockets driven by the hydraulic driving apparatus with the switching valve, does not disclose or suggest a wheeled vehicle employing the hydraulic driving apparatus with the switching valve.
Furthermore, the disclosed switching valve, when it is located in its neutral position, blocks the pair of oil passages on the hydraulic motor side, thereby blocking the hydraulic motor. In this situation, even if the ground is rough and the right and left drive axles are subjected to considerably different ground resistances, the blocked hydraulic motor does not rotate and does not absorb the resistant difference between the axles. Therefore, excessive stress is applied onto the hydraulic motor, the axles, and a drive train therebetween, and also, the ground surface such as turf may be damaged.
Another disadvantage of the disclosed switching valve which blocks the hydraulic motor when it is set in neutral, is that if the steering operation means is operated for turning right or left of the vehicle from the straight traveling setting, and the switching valve is switched from the neutral position to either the forward or rearward traveling position, the blocked hydraulic motor delays its start of rotation, thereby reducing the steering response.
If the hydraulic motor, when the switching valve is located in the neutral position, can be softly rotated, the above-mentioned problem is solved. However, the sure straight movability of the vehicle, when the steering operation means is set in the straight traveling position, is spoiled.